Bituminous nonskid pavement



Patented Nov. 21, 1939 UNITED STATES,"

BITUMINOUS NONSKID PAVEMENT Joseph H. Conzelman, Birmingham, Ala.,assignor to Alabama Asphaltic Limestone Company, Birmingham, Ala, acorporation of Alabama No Drawing. Application July 2, 1937,

Serial No. 151,645

5 Claims.

This invention relates to a new and novel cold laid bituminous pavementand the method of laying it and has for its object to produce a highlytractive nonskid surface and to retain therewith the desirable featuresof good paving mixture design.

Where properly designed cold laid bituminous paving mixtures are used,either for full-thickness pavements or seal coats, it has been foundthat the resulting surface offers a low tractive resistance, andespecially is this true if a slight excess of bituminous material isused in the dense low void mixture of conventional design.

In order to make road and street surfaces as nonskid as possible therehas been considerable deviation from good paving mixture design byincorporating in the mixture a high percentage of voids, as it has beenfound that high void mixtures offer better tractive resistance thandense well designed bituminous mixtures. However, these open mixturesare not satisfactory in that they often lack toughness and stability andare further objectionable for the reason that the excessive voids in themixture permit more or less rapid disintegration through thedeteriorating action of moisture, air and light which penetrate it.

Seal coats on bituminous pavements or base first consisted ofapplications of bituminous material covered with crushed stone, slag,gravel or sand. Such seal coats contain excess of aggregate in spotsthat may ravel and excess of bitumen in others that result in slipperyareas. These facts have led to the development of premixed seal coatscontaining a uniform amount of bitumen and offering possibilities forcorrectly graded aggregate. As in the case of most cold laid bituminousmixtures, premixed seal coats or seal coats mixed in place have beenopen so that necessary stability through evaporation oflique fier usedcan be developed and also for the purpose of making the surfacesnonskid- Such seal coat mixtures do not protect the surface or base theyare laid on from moisture. They are subject to rapid deterioration fromthe effects of the moisture, light and air that their high void contentallows to penetrate throughout the thickness of material laid.

I have found that the advantages of both the dense and the open mixturescan be combined in a cold laid surface or seal coat by spreading all buta predetermined portion of the dense mixture, then adding to the balancea sufiicient quantity of hard, abrasive aggregate to so change thecharacter of the remaining portion that it will have a coarser texture.The body of the surface constructed in this manner will be tough,stable, and

impervious to moisture and the top portion containing the abrasive agentwill be nonskid but still will contain suflicient well designed mixtureto prevent the rapid weathering and raveling prevalent in open typemixtures.

In some localities bituminous coated broken stone, usually graded from Ato in size, has been distributed over the surface of hot mix pavementsduring the process of rolling. These chips are then rolled, theintention being to anchor them in the hot, plastic surface to increasethe frictional resistance of the pavement. Gen-. eral practice has beento use these stone chips in the quantity of from seven ('7) to twelve(12) pounds per square yard.- This is sufiicient quantity to result inpockets of chips in some areas Where only the bottom particles ofaggregate are keyed into the hot mix surface. No effort is made to mixthe bituminous coated chips with the hot surface mixture. The resultingsurface contains a large percentage of open or high void spots in thepavement where chips segregate that hold moisture, with the. result thatthe chips not only disintegrate under traific but also throughatmospheric action lose their bituminous coating and ravel out undertraflic. Under this method of securing a non-skid surface it isdifficult to anchor the nonskid aggregate uniformly over the pavementsurface. The aggregate is rolled on the surface and is not mixed withthe surface mixture as my invention contemplates.

Mixing this abrasive aggregate with the material in the upper portion ofthe surface mixture as contemplated by my invention can be accomplishedin various ways. Mixtures'that can be spread with a motor patrol, orself-propelled road grader, may be bladed into place with the exceptionof the last five (5), ten (10) or twenty pounds of material to bespread. To this remaining windrow of material may be added the desiredquantity of the abrasive aggregate, preferably coated with bituminousmaterial limited in amount to that which will allow the material toseparate into individual particles during the subsequent mixing process.The abrasive aggregate can then be thoroughly mixed with the remainingsurface mixture and the resulting mix spread uniformly over the pavementsurface by means of the motor patrol. The major portion of the surfaceso laid would be dense, water resistant, well designed bituminousmixture. The surface in contact with Vehicles would contain nonskidaggregate held firmly in place by the properly designed mixture that hasbeen mixed with it.

There are also many other ways that this mixing of abrasive aggregatewith the upper portion of cold laid surface mixture could beaccomplished. The abrasive material may be mixed with the upper portionof the surfacing mixture with rakes, or it may be distributed over thesurface of the cold laid mixture after it is spread and before it isrolled, and can be mixed with the upper portion of surface by use of adrag preferdisplace the upper particles of surface mix, mixing theseparticles with the abrasive aggregate. The intent of this invention isto mix abrasive aggregate, preferably having a light bituminous coating,with the upper portion of a dense cold laid paving mixture in suchmanner that the abrasive aggregate is mixed with and not merely imbeddedin the surface mix.

As an example, in laying a forty pound per square yard seal coat orleveling course of Alabama asphaltic limestone cold mixture thematerial, spread by a motor patrol, would be bladed into place, buildingup the surface by wasting the material out light lightly undersuccessive trips of the blade until approximately ten (10) pounds persquare yard of the cold mix remained in the windrow. To this windrowwould then be added a suilicient amount of the abrasive aggregate to addten (10) pounds per square yard of aggregate to the material in thewindrow. To be most effective the nonskid aggregate should be a hardstone, slag, gravel, or sand. The maximum size particles wouldpreferably be This aggregate to facilitate bonding with the rock asphaltmixture is coated with as heavy a film of bitumen as the aggregate usedwill held without bonding the particles together. For this example aspecially prepared hard, dense slag would be used, graded from to 10mesh sizes and'coated with from three (3) to four (4) percent of asphaltcutback. When the abrasive, or nonskid aggregate has been added to thewindrow it is thoroughly mixed with the cold laid asphaltic limestone bymeans of the self-propelled motor patrol and the spreading of thesurface completed. Rolling of this surface can be carried on during thespreading of the mixture or after spreading has been completed.

It is obvious that any aggregate hard enough to stand abrasion betterthan the softer grade of paving aggregates would serve as a nonskidaggregate in the example just described if the nonslrid aggregate wasgraded satisfactorily. Preferably the aggregate used should not containparticles larger than in size and should not be graded so that itcontains a low percentage of voids. Pea gravel; concrete sand; finersand if at least fifty (50) percent is one commercial screen size;crushed hard stone, gravel or slag with the 50 mesh and liner particlesremoved if more than twenty (20) percent of such particles are present;any of these if mixed as described with the upper portion of a wellgraded cold mix will increase the frictional resistance of the resultingpavement.

By the expression well balanced dense aggregate employed in the claims,is meant an aggregate which has such a distribution of grades or sizesof particles that there are sufficient progressively finer grades tosubstantially fill the voids of the coarser ag egate.

By the expression unbalanced relatively open aggregate employed in theclaims, is meant an aggregate having a disproportionate amount of somesize or sizes of aggregate other than the finest so that substantialvoids are present.

I claim as my invention:

1. A bituminous pavement comprising a compacted layer of bituminousmixture having a lower portion containing a well balanced denseaggregate, the particles of which are coated and 'ably containing pegsor teeth that agitate and bonded together with bitumen to form a stablewater sealing strata and an upper portion containing an unbalancedrelatively open aggregate the particles of which are coated and bondedtogether with bitumen to provide a nonskid trac tive surface, said upperportion forming a substantial part of said layer and merging into andbeing integral with said lower portion.

2. A bituminous pavement comprising a compacted layer of bituminousmixture having a lower major portion containing a well balanced denseaggregate with the particles thereof coated and bonded together withbitumen to form a stable water sealing strata and an upper portioncontaining said aggregate in admixture with an abrasive aggregate andproviding an unbalanced relatively open resulting aggregate, theaggregate particles of said upper portion being coated and bondedtogether with bitumen to provide a nonskid tractive surface, said upperportion being a substantial part of said layer and merging into andbeing integral with said lower portion.

3. The method of laying a bituminous pavement which comprises spreadinga main layer of well balanced dense aggregate having the particlesthereof coated with bitumen, forming a second layer of unbalancedrelatively open aggregate, having the particles thereof coated withbitumen, over said main layer prior to substantial compacting of saidmain layer and thereafter compacting said layers to produce a bituminouspavement having a lower portion forming a stable water sealing strataand an upper portion of substantial depth merging into and integral withsaid lower portion and providing an antiskid tractive surface.

4. The method of laying a bituminous pavement, which comprises spreadinga layer of well balanced dense aggregate having the particles thereofcoated with bitumen, mixing an abrasive aggregate having particlescoated with bitumen with the upper portion of said layer prior tosubstantially compacting the same to produce a strata of substantialthickness containing an unbalanced relatively open aggregate, andcompacting said layer to produce a bituminous pavement having a lowerportion forming a stable water sealing strata and an upper portion ofsubstantial thickness merging into and integral with said lower portionand providing an antiskid tractive surface.

5. The method of laying a bituminous pavement, which comprises spreadinga main layer of bituminous mixture containing a well balanced denseaggregate having the particles thereof coated with bitumen, mixinganother aggregate having particles coated with bitumen with anadditional amount of said mixture to produce another bituminous mixturecontaining an unbalanced relatively open aggregate, spreading a layer ofsaid other mixture over said main layer prior to substantial compactingof said main layer and thereafter compacting said layers to produce abituminous pavement having a, lower major portion forming a stable watersealing strata and an upper portion of substantial thickness merginginto and integral with said lower portion and providing an antiskidtractive surface.

JOSEPHH. CONZELMAN.

